Presently, as next-generation communication standards of LTE (Long Term. Evolution), 3GPP (3rd Generation Partnership Project) is developing specifications designed to sophisticate LTE-Advanced. In LTE-Advanced systems, carrier aggregation (CA) technique is introduced to achieve a higher throughput than that of LTE systems while ensuring backward compatibility with the LTE systems. In the carrier aggregation, a component carrier (CC) having the maximum bandwidth of 20 MHz supported by the LTE systems is used as a basic component, and it is designed to achieve communication in a broader band by using these multiple component carriers simultaneously.
In the carrier aggregation, user equipment (UE) can use multiple component carriers simultaneously to communicate with a base station (evolved NodeB: eNB). In the carrier aggregation, a highly reliable primary cell (PCell) to ensure connectivity to the user equipment and a secondary cell (SCell) additionally configured for the user equipment in connection with the primary cell are configured.
The primary cell is similar to a serving cell in the LTE systems and serves as a cell to ensure connectivity between the user equipment and a network. On the other hand, the secondary cell is a cell configured for the user equipment additionally to the primary cell. Addition and deletion of the secondary cell are performed with an RRC (Radio Resource Control) configuration.
In the carrier aggregation up to LTE Release 10 (Rel-10), as illustrated in the left side in FIG. 1, it is defined that user equipment uses multiple component carriers served from a single base station to conduct simultaneous communication. Meanwhile, in Rel-12, the carrier aggregation in Rel-10 is further extended, and as illustrated in the right side in FIG. 1, dual connectivity where the user equipment uses multiple component carriers served from multiple base stations to conduct the simultaneous communication is discussed. For example, if all component carriers cannot be accommodated in a single base station, it is considered that the dual connectivity can be effectively utilized to achieve a throughput nearly equal to that in Rel-10.
In the dual connectivity, as illustrated in FIG. 2, bearer splitting where user equipment (UE) splits a single EPS (Evolved Packet System) bearer or packet sequence in a predefined manner and uses component carriers served from multiple base stations (eNB#1, eNB#2) to transmit the respective split packet sequences simultaneously is discussed. Specifically, as illustrated, the user equipment splits the to-be-transmitted EPS bearer into packet sequences destined for eNB#1 and eNB#2 in a certain ratio (eNB#1:eNB#2=4:3 in the illustrated example) and transmits the respective split packet sequences to the base stations eNB#1 and eNB#2 via component carriers CC#1 and CC#2, respectively. Upon receiving the split packet sequence via CC#2, the base station eNB#2 serving as a non-anchor node forwards the received packet sequence to the anchor base station eNB#1. Upon receiving the packet sequence forwarded from eNB#2, eNB#1 reorders the packet sequence received via CC#1 and the packet sequence received from eNB#2 to reconstruct the packet sequence from the user equipment and forwards the reconstructed packet sequence to a core node (CN).
See 3GPP TR36.842 “Study on Small Cell enhancements for E-UTRA and E-UTRAN; Higher Layer aspects” for detail, for example.